There is no easy answer to this because it depends on what you are trying to do. Buying fairly developed players who won't necessarily improve much versus younger players with potential makes a difference.
Your scouts can only tell you so much. They can give you background information that simply looking at the player won't - scouting reports hint at things like injury proneness, consistency, and adaptability. They can compare the player to a current player and give you an idea of who is better. They can let you know if the player is likely to develop. But they can't tell you how the player would fit into your current tactical vision, let alone what you want to evolve it into. So relying on the scouts entirely is basically giving away one of your roles as a manager.
Who should you target? Players that make your side better. Players that fit how you want to play. The "better" player is very subjective, because its all about how a player fits. You might sign a wonderful, creative mastermind but if you have a hard-working side that requires everyone contribute in all phases, they might not fit very well. A big, strong target man with great finishing might not be ideal for a side based around possession and movement. They can work, but they may not be what works best.
Look at the players you have. Consider how well they perform. Then ask why they perform that way. Are you using them to their strengths? Are they performing as well as they could? If someone doesn't perform, or doesn't do so consistently, why not? Is it an issue with the player, the tactic, or the opposition?